A high-alloy austenitic CrMnNi steel was deformed at temperatures between 213 K and 473 K (À60°C and 200°C) and the resulting microstructures were investigated. At low temperatures, the deformation was mainly accompanied by the direct martensitic transformation of c-austenite to a¢-martensite (fcc fi bcc), whereas at ambient temperatures, the transformation via emartensite (fcc fi hcp fi bcc) was observed in deformation bands. Deformation twinning of the austenite became the dominant deformation mechanism at 373 K (100°C), whereas the conventional dislocation glide represented the prevailing deformation mode at 473 K (200°C). The change of the deformation mechanisms was attributed to the temperature dependence of both the driving force of the martensitic c fi a¢ transformation and the stacking fault energy of the austenite. The continuous transition between the e-martensite formation and the twinning could be explained by different stacking fault arrangements on every second and on each successive {111} austenite lattice plane, respectively, when the stacking fault energy increased. A continuous transition between the transformation-induced plasticity effect and the twinninginduced plasticity effect was observed with increasing deformation temperature. Whereas the formation of a¢-martensite was mainly responsible for increased work hardening, the stacking fault configurations forming e-martensite and twins induced additional elongation during tensile testing.

Plastic deformation of highly alloyed austenitic transformation‐induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ɛ‐martensite within the original austenite. The ɛ‐martensite is often described as a phase having a hexagonal close‐packed crystal structure. In this contribution, an alternative structure model is presented that describes ɛ‐martensite embedded in the austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X‐ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and ɛ‐martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X‐ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.

Dense TRIP‐matrix composites containing 5 vol.% Mg‐PSZ as reinforcing phase were produced by employing the spark plasma sintering technique. A continuous and seamless interface between the ceramic particles and the steel matrix was achieved. Compression tests revealed better mechanical properties of the 5 vol.% Mg‐PSZ‐TRIP steel composites in comparison with both, pure and Al2O3 reinforced TRIP steel. The underlying deformation mechanism within the austenitic matrix entailed a pronounced martensite formation. An additional phase transformation was observed within the ZrO2 particles. The enhanced mechanical properties of the 5 vol.% Mg‐PSZ composite are dedicated to the transformation strengthening of the ceramic particles. Finally a model of the reinforcing mechanism is proposed.

At different temperatures ranging from ‑60°C to 200°C a cast CrMnNi-TRIP steel was deformed by uniaxial tension. The resulting microstructure was investigated using XRD, EBSD and LOM. The correlation of the phase transformation with the deformation temperature was examined. Depending on temperature, a transition in the deformation mechanisms was observed. Starting with the generation of deformation bands, accompanied by martensitic phase transformation, followed by twinning, the deformation mechanism turned to conventional dislocation glide with raising temperature. Between -60°C and 20°C the TRIP (TRansformation Induced Plasticity)-effect is the dominating deformation mechanism, whereas between 20°C and 200°C the TWIP (Twinning induced plasticity) effect is observed. The geometrical arrangement of martensite within the microstructure is considered within this study. The amount of α'-martensite is mainly responsible for the hardening rate and the resulting mechanical properties.

No abstract